Welding apparatus



sept. 27, 196e Filed Aug. 14, 1965 J. D. HELMS WELDING APPARATUS 4Sheets-Sheet l Sept. 27, 1966 J. D. HELMS WELDING APPARATUS 4Sheets-Sheet 2 Filed Aug. 14, 1965 FIGB.

FIGQ.

Sept. 27, 1966 J. D. HELMs 3,275,790

WELD ING APPARATUS Filed Aug. 14, 1963 4 Sheets-Sheet 5 Houde faeJ I kan`fof l A l WELD VOLTAGE ERROR PULSE E AMPLIFIER OOMPARATOR AMPUFIERREFERENCE VOLTAGE `d223 Sept. 27, 1966 J. D. HELMs WELDING APPARATUS 4Sheets-Sheet 4 Filed Aug. 14, 1963 United States Patent() 3,275,790WELDING APPARATUS .lohn D. Helms, Dallas, Tex., assgnor to TexasInstruments Incorporated, Dallas, Tex., a corporation of Delaware FiledAug. 14, 1963, Ser. No. 303,213 6 Claims. (Cl. 219-78) This inventionrelates to welding apparatus, and more particularly to welding apparatusadapted among other uses to Weld the leads of microminiature electroniccomponents such as semiconductor networks and the like to the conductiveportions of printed circuit boards. Y

Among the several objects of the invention may be noted .the provisionof a parallel-gap wellder which facilitates the application andinterconnection of microminiature electronic circuit components toprinted circuit boards; the provision of welding apparatus havingimproved mechanical and electrical characteristics which make possiblethe Welding of very ne conductive leads or wires to thin conductiveportions on a nonconducting substrate of glass, ceramic, epoxy, or thelike, without damaging the leads, said conducting surface portions, orthe substrate; the provision of welding apparatus of the yclassdescribed wherein the welding current through the workpiece being weldedis accurately controlled during a welding cycle; the provision ofwelding apparatus where-in a preresistance test on the wor-kpiece isemployed to determine whether its electrical resistance falls within apreselected tolerance range, and wherein a weld cycle is prevented ifthe resistance of .the workpiece is not within said range; the provisionof welding apparatus Of the class described wherein pre-resistance an-dpost-resistance measurments are made to determine whether the electricalresistance of the workpiece is within a tolerable range both before andafter a welding cycle; the provision of a conveniently operable Weldermechanism adapted to apply replaceable electr-odes to the work atselectively predeterrmined4 equaloperating pressures; the provision of amechanism of the class described which will effectively apply itselectrodes to coplanar or noncoplanar surfaces; the provision of suchmechanism in which arrangements are made for accurately and convenientlyadjusting the electrode spacing over a substantial range; the provisionof such .a mechanism including means for preventing eX- cess contactpressure of the electrodes; the provision of welding apparatus whereinthe gap between the weld electrodes insures that the welding area isclearly visible during a welding cycle, and wherein ease of access tothe work beneath the electrodes is maintained during the cycle; theprovision of apparatus of the class described wherein parallel electrodetips are employed, eliminating the necessity of realignment due toelectrode wear; and the p-rovision of welding apparatus wherein thequality of the weld joint is accurately controlled and wherein the weldprocess can be easily automated. Other objects and features will be inpart apparent and in part pointed out hereinafter.

The invention accordingly comprises the constructions and circuitshereinafter described, the scope of the invention being indicated in thefollowing claims.

In the accompanying drawings in which two of various possibleembodiments of the invention are illustrated,

FIGURE 1 is a diagrammatic side elevation of certain .mechanicalelements of the invention, parts being broken "ice FIGURE 5 is a crosssection illustrating certain pedaloperating means;

FIGURE 6 is an enlarged side elevation of an electrode and its support,parts being shown in section;

FIGURE 7 is a section taken on line 7--7 of FIG- URE 6;

FIGURE Sis a front view of a pair of electrodes shown as beingpappliedto coplanar Welding areas;

FIGURE 9 is a front view of a pair of electrodes shown as being appliedto noncoplanar welding areas;

FIGURE 10 is a block diagram illustrating a preferred electrical controlsystem for the welding apparatus of FIGURES 1-9;

FIGURE 11 is a circuit diagram illustrating the elec- .trical componentsof the FIGURE 10 system and their interconnection; and

-FIGURE 12 is a block diagram illustrating a second control system forthe welding apparatus of FIGURES 1-9, mechanical portions of the systembeing shown diagrammatically.

Corresponding reference characters indicate correponding partsthroughout the several views of the drawings.

Refer-ring now more particularly to FIGURE 1, the-re is shown at numeral1 a base which carries a suitable worktable or insulated weldingplatform 3 for supporting work to be welded. On the b-ase 1 is a housing5, open at the front, as shown at 7. On the base 1 is a bracket 9 whichsupports a movable hydraulic cylinder 11. Support is effected by asliding t between a hole 10 in the bracket 9` and a downward cylindricalextension 12 of the cylinder 11.

yIn the cylinder 11 is a movable piston 13 which is sealed duringmovements by a flexible diaphragm 1S marginally held by the cylinder.Carried on the piston 13 is a piston rod 17. This passes through thelower extension of the cylinder 11. At its outer end rod 17 is attachedat 19 to a vertically movable bracket 21. Bracket 21 forms part of acarrier member 23 which is 'vertically bored .to receive a post 25 forvertical sliding 'vertical rods 33; Slidlable for a limited distance oneach rod 33 is a sleeve 35. Within each sleeve 35 is an electricallyinsulating bushing 37, flanged at its opposite ends, as shown at 39.

Extending forward from the rib portion 29 is a bracket 41 which supportsa rotatable crosspin 43 (FIGURES 2 and 4). The pin 43 is held captiveaxially by set screw collars 45. It carries a calibrating knob 47. Underthe knob is a calibration dial 48 which is attached to one of a pair ofarms `59 (further to be described below).

The pin 43 carries threads 49 and 51 at its opposite ends. These are ofopposite pitches and carry nuts 53. The arrangement is such that whenthe pin 43 -is turned, the nuts 53 will recede or approach relative toone another, depending upon the direction in which the knob 47 isturned. Attached to and extending up and down from each nut 53 is a pairof collinear pins 55 which have slidable engagement with insulatingbushings 57. The rbushings 57 are located in .arms 59, which extendforwardly from the vertically movalble :members 35 respectively. Thepurpose of this arrangement is to permit two types of movement -for thearms 59; rst, opposite transverse swinging arm movements around thecenters of the guide rods 33, so that welding rods (to be described) mayhave the space between them adjusted by turning knob 47; and second,opposite vertical :movements of the larms 59 (with the sleeves 35 fromwhich they extend) so that compensations can be effected in the contactplanes of said welding rods.

The Iamount that the sleeves 35 of the arms 59 may move up and down ontheir pins 33 is determined by the clearances shown at 61 (FIGURES 1 and2).

Extending up from the bracket 41 is a lug 63 forming a bearing f-or oneend of a rocker pin 65. The other end of this pin is borne in a hole 67in the rib 29 of carrier 23. Rotatable on pin 65 is a compensating beamor lever 69 which has a rounded head 71 at each end. Each head 71 islocated in an opening 73 of` one of the arms 59. Insulating bushings 75in the openings 73 surround the heads 71. If desired, instead ofemploying insulating bushings 75, the beam 69 might be made ofinsulating material.

At numerals 77 are shown the welding rods or electrodes (abovementioned), each of which is located in an opening 79 of a downwardlydirected finger part 81 of loosely but captive in a suitable cage 85located in finger 81. For assembly purposes, an -opening 87 is Iprovidedextending across rod opening 79, for the introduction of the cage 85.Before the cage and ball are introduced, a wedging member 89 ofhour-glass shape is threaded into -an opening 91 extending crosswise -ofthe finger part 81. This wedging member 89 contains a screw-driver slot93, whereby it -may be rotated so as to wedge the ball 83 into holdingcontact with a welding rod 77 in opening 79. It will be lunderstood thatother `appropriate means may be used. Afor clamping the electrodes inposition in their respective arm portions 81.

The lower en-d of each electrode 'has a welding tip 95 (FIGURES 6, 8 and9). The opposed faces or surfaces of electrodes '77 are lparallel (seeFIGURES 8 and 9) so that the tips 95 are parallel. This eliminates thenecessity of -realignment of the electrodes due to electrode wear. Atits upper end each electrode carries an exteriorly threaded socketmember 97 for the reception of a jack 99 forming the end of a lead wire101. The jack 99 is held in place by la cap 103, threaded to the socketmember 97. The purpose of wire 101 will be described below.

In FIGURE is shown pedal-operating means com- Iprising a lbase 105 towhich at 107 is pivoted a foot pedal 109. Foot pedal 109 carries abracket 111 forming a yguide for -a slider member 113. The position ofthe slider is cont-rolled by an .adjusting screw 115. Pivoted at 117 tothe slider 113 is a piston rod 119. This is connected with a plunger 121located in a flexible sealing bellows 123 in a hydraulic cylinder 125.The plunger 121 and bellows form a piston in the cylinder 125, operableby the piston rod 119 when the pedal 109 is rotated clockwise. yByadjusting the slider 113 on the bracket 111, the position of pivot 117relative to the pedal 109 is changed and by this means the stroke of theplunger may be changed, as desired.

The cylinder 125 has a rigid extension 127 pivoted at 129 to a bellcrank 131. The bell crank is pivoted at 133 -to yan appropriate support135 on the base 105 and engages a plunger 137, backed by an overloadspring 139. The purpose of the plunger 137 and spring 139 is to provideIa release of the bell crank 131 when hydraulic pressure in a cylinder125 approaches that corresponding to a selected safe limit. The motionof the pedal 109 is also limited by adjustable stops 141 and 143.

A flexible hydraulic pressure line 145 leads from the cylinder 125 tothe head 147 of cylinder 11 (FIGURES 1, 3 and 5). Thus when the pedal109 is rotated crossw'ise l(FIGURE 5) hydraulic pressure is introducedinto the cylinder 11 so as to push down the piston rod 17, bracket 21and carrier Iparts connected therewith. Return of fluid to cylinder 125is provided for by return action on piston 13 of a spring 149 when yfootpressure is taken from the pedal 109.

Referring again to FIGURES 1 and 3, the top of the housing 5 carries almotion transmission box 151 operative between a calibrated dial 153 anda gear train 157 for; turning a threaded vertical post 155. On the postis a nut 159 which Iis vertically movable when the post 155 is turned.An elongate bracket 160 (shown in FIGURE 3) prevents the nut fromturn-ing while permitting its axial movement (FIGURE 3). The nut iscoupled to a lever 161 through a tension spring 163, adapted to behooked into various holes 165 of the lever 161. The lever is pivoted at167 to the top of the housing 5 and is engaged on its -underside at 169by the head portion 147 of the cylinder 11. The lever also carries a lug171, engageable with the operating button of a normally open firingswitch 173. This switch is carried Afrom the top of the housing 5 by abracket 174. By turning the calibrated knob 153 and thus adjusting theelevation of the nut 159, various clockwise movements of force may beobtained on the lever 161 from spring 163.

At numerals are shown welding-current leads which extend to 'bus Ibars177 carried on an insulated plate 179 located on base 1. Flexi-blecurrent leads 181 extend from the bus bars 177 to the sleeves 35,whereby current from a capacitor-discharge type power supply (describedhereinafter) is conducted through arms 59 to the Welding rods orelectrodes 77 As noted above, the Welding apparatus of this invention isadapted among other uses to weld the leads of microminiature electroniccomponents such as semiconductor networks (SCN) or integrated modules tothe conductive portions or plating of printed circuit boards. Indescribing the operation of the mechanical portion of the apparatus, itwill be assumed that this workpiece, i.e.,

the printed circuit board, and the SCN are in proper position on table3. These are shown in FIGURES 8 and 9, the SCN (or a portion thereof) atreference numeral 183 and its conductive lead at 185. The conductiveportion or plating of a printed circuit board (indicated at 187) isshown at numeral 189, and the nonconducting substrate thereof isindicated at numeral 191.

Operation of the mechanical portion of the apparatus is as follows:

The Calibrating knob 47 is turned, thus driving the nuts 53 on threadedcrosspin 43 either together or apart. This swings the carriers 35oppositely around the rods 33, whereby the desired distance is obtainedbetween the Welding tips 95. The calibration at the knob 47 indicatesthese distances which may be in the range of O to 40 mils.

Next, foot pressure on pedal 109 rotates it clockwise,

vwhich squeezes Huid under pressure out of the cylinder 125 through line145 to the head 147 of cylinder 11. Since at this time the electrodes donot contact their work, the piston 13 responds by moving downwardly moreor less freely, although against return action of the spring 149. Thismoves the bracket 21 down along with the assembly of parts 23, 31 and41. These slide down on the post 25. This moves down the bracket 41 andits lug 63 which in turn carries down the pin 65. Pin 65 carries downwith it the compensating lever or beam 69 which in turn carries down thesleeves 35, arms 59, fingers 81 and welding rods 77. Before any contactis made by electrodes 77 with the work, the two carriers 35 may or maynot remain at about the same level on their posts 33, as shown in FIGURE2. If the electrodes 77 make contact with the work in one plane, asshown in FIGURE 8, they remain or seek a level. If they contact the workin different planes, then the state of affairs shown in FIG- URE 9occurs. In this case one (the right-hand) carrier 35 stops. Then as theparts descend further, the compensating beam 69 rocks, forcing down theother carrier 35 and taking with it the left-hand electrode 77. It willbe apparent that the sliding fits of the pins 55 of vnuts 53 in thebushings 57 in arms 59 allow for com pensating action between the arms59 and the space-adjusting crosspin 43, as required. In any event, whenthe electrodes 77 make contact with the work, they exert equalpressures, either as shown in FIGURE 8 or FIG- URE 9. Moreover, areaction will be set up by them through the compensating beam 69 to theparts which slide on the post 25. These parts, as above noted, includethe bracket 21. As a consequence the pressure in the cylinder 11 cannotpush the piston 13 down any further. Pressure on the head 147 continues.Thereupon the cylinder 11 is pulled up, its part 12 sliding in the hole10 of the bracket 9. At this time the parts connected with bracket 21 donot move, pressure on the work being maintained by the welding tips 95.Since the cylinder head 147 engages under the lever arm 161 at 169, thistends to rotate the arm anticlockwise against the adjusted tension ofthe spring 163.

Ultimately the lug 171 causes firing switch 173 to close. As explainedhereinafter, this actuates a relay which in turn causes a capacitor tobe discharged through the leads 175, the electrodes 77 and the work. A.pulse of welding current passes from one of the welding electrodes 77to the other through the parallel branches or conductive paths formed bythe SCN lead 185 and the conductive portion or welding tab 189 of theprinted circuit board. As explained hereinafter, the peak amplitude ofthe pulse voltage is accurately controlled during the weld cycle so asnot to exceed a preestablished level. This is an important feature ofthis invention, as will be described in connection with FIGURES 10-12.

Because, with voltage control of the welding pulse, there is a constantand equal voltage across the two parallel paths, 185 and 189, the powerdissipation in each path is an inverse function of its resistance. Themost signicant results of this relationship are that the powerdissipated in a conductor to be welded is a direct function of its crosssection, and any sudden increase in the resistance of parallel circuit185 and 189 will not result in a catastrophic failure as would be thecase-were a constant current source being used. The weld joint thusformed does not extend over the full width of the gap due to theheatsink action of the electrodes. The high thermal conductivity ofthese electrodes (which are preferably made of copper) limits fusion toapproximately 80% of the electrode gap. This heatsink effect isdesirable because it eliminates fusion near the electrode tips whichotherwise would result in the tips sticking to the welded surface.

Since switch 173 is actuated at a preselected pressure between theelectrode tips and the workpiece, a high uniformity of welds may beattained. Acceptable welds joining SCN networks to so-called G-11 baseprinted circuitry have been made over a total pressure range from 1.5 to3.5 pounds. By total pressure is meant the sum of the pressure exertedby the electrode tips. This wide range of acceptable pressure ispossible because the weld is formed between the electrodes instead ofbeneath the electrodes. In any specifick application, the only limitsimposed on the electrode pressure are (1) the total pressure must besuflicient to break down surface oxidation between the network lead andthe Welding tab, and (2) this pressure must be less than a value whichwould tend to weaken the bond between the welding tab and the substrate.

The electrode gap (defined as the horizontal distance between theinterfaces of the two electrodes tips) is a critical parameter inobtaining optimum weld joints. The electrode gap controls'the mass offusion nugget formed by the welding current pulse, large gaps resultingin the formation of large nuggets. The higher energies required to formlarge nuggets burn the board beneath the tab and weaken the organic bondbetween the substrate and the plating; while gaps which are too smallresult in a joint which is mechanically weak, due to incomplete fusion.Different applications will Irequire different gap settings, and theaccurate control 4of the gap by means of knob 47 and its associatedcomponents constitutes an important feature of this invention.

After firing, the pedal 109 is permitted to return anticlockwise to itsinitial position, as shown in FIGURE 5. The spring 149 then expands,thereby returning cylinder 11 to its initial position and causing thepiston rod 17 to lift bracket 21 and the parts including the electrodesto their elevated positions away from the work. In the process, fluidows back through the line to the cylinder 125. In view of the above, itwill be apparent that the sealed piston 13 and cylinder 147 constituteexpansible uid chamber means having relatively movable parts, one ofwhich (the piston) is connected to the electrode carrier means to movethe electrodes to and against the work. The other part (head 147) isarticulated to the lever or arm 161 which provides adjustable reaction(from spring 163) to determine the electrode contact pressure at theinstant of welding.

A preferred embodiment of the electrical control portion of the weldingapparatus which functions to provide selective energization of the weldelectrodes and critical regulation of the current pulse through theworkpiece during a welding cycle |will now be considered in connectionwith FIGURES l0 and 11. Referring now to FIGURE 10, weld electrodes 77are shown in contact with lead which is to be welded to the tab orsurface 189. The flexible leads 181 are shown interconnected lto theelectrodes 77. These supply power from a capacitor-discharge type powersupply consisting of a welding transformer 201 having a primary winding205 and a secondary winding 203, a regulating resistance 207, a weldingrelay 209 (which, as explained hereinafter, is under the control offiring switch 173), and a capacitor 211. The latter is selectivelyconnected by relay 209 to a source of -charging current, for example a110 v. D.C. power supply connected at point 212. A shunt 213, which, asexplained hereinafter, consists of a variable and controlled impedancein the form of one or more transistors, is connected in series withresistor 207, and in parallel with primary winding 205.

In the quiescent state, the movable contact of relay 209 is in theposition illustrated wherein it interconnects the D.C. power supply withcapacitor 211, charging the latter to the 110 v. level. Actuation oftiring switch 173 causes energizaticn :of relay 209 which in turnconnects capacitor 211 to .resistor 207 thereby providing a dischangepath for this capacitor through resistance 207, and through primarywinding 205 and shunt 213. The pulse through winding 205 is coupled tosecondary winding 203 and applied to electrodes 77. A typical pulseapplied across the workpiece will have an amplitude of from .5 to .75v., for example, and a duration of 2-3 milliseconds.

Because the impedance between electrodes 77 when they are not -incontact with the workpiece is consider-ably higher than the resistanceof the workpiece between electrodes 77, and since the latter is quitelow, for example, 3 milliohms, the capacitor-discharge power supply isessentially a constant-current source for small changes in theresistance of the workpiece. Because of this, the heat generated duringthe welding cycle (which is equal or proportional to 12R) varies withthe resistance of the workpiece. And the thinner workpieces which areless able to stand increased heat have higher resistances resulting inhigher 12R heat generation. For example, if a lead having a thickness of4 mils has a resistance of 3 milliohms, a 2-mil lead would have a.resistance of 6 milliohms; rand the lead having lhalf the volume wouldin effect receive double the heat input. The present invention obviatesor overcomes the burn-out problem by including a :feedback circuit whichprovides a dynamic control during a welding cycle which maintain-s thevoltage across the workpiece, and hence the -current therethrough, at avalue less than a preestablished safe level.

In this controlled feedback system, the lead receives heat in proportionto its volume because this system sets the voltage level across the tipsas the weld is taking place.

This feedback circuit includes a pulse weld amplifier 219, the input ofwhich is connected by conductors 101 to the welding electrodes 77. Asshown rnost clearly in FIGURE l, conductors 101 are connected directlyto the weld electrodes immediately adjacent the workpiece. This is animportant feature of the present invention which increases or enhancesthe signal-to-noise characteristics by reducing the total D.C.resistance between the sensing conductors 101. Pulse weld amplifier 219is a D.C. coupled amplifier having a gain of approximately 6 to 10. Thisamplifier functions to bring the weld pulse up from an increment of .5v. to from 3 to 5 v. The output of amplifier 219 is applied to one inputof a voltage comparator or differential amplifier 221, -the Iother inputof which receives a reference voltage from a reference voltage generator223. An output of comparator 221 occurs when the input from amplifier219 exceeds the reference voltage supplied by generator 223, with theamplitude of the output being a function of the difference between thetwo inputs. The output of comparator 221 (when one occurs) is fed to anerror amplifier 225 which in turn controls the value of the shuntimpedance 213 to bring the weld pulse back to its proper value. It will.bc appreciated that regulating resistor 207 and shunt 213 constitute avoltage divider network, the output of which (applied to primary Winding205) is taken across shunt 213.

A sequence of operation of the FIGURE control is as follows: Initiallyrelay 209 is deenergized, and the movable contact thereof interconnectscapacitor 211 with point 212 causing this capacitor to be charged.Actuation of firing switch 173 energizes relay 209, thereby moving thecontact thereof to interconnect capacitor 211 with resistor 207. Thisprovides 1a discharge path for this capacitor through winding 205 andshunt 213. Transformer 201 couples the resulting pulse to electrodes 77causing a short duration Weld pulse to be conducted through theworkpiece. Pulse weld amplifier 219 and voltage comparator 221continuously monitor this pulse during a weld cycle, and if the pulseincipiently exceeds a preestablished level as determined by the settingof generator 223, these components provide `a control signal to shunt213 which Varies the impedance characteristic thereof to bring the weldpulse below this preestablished value. Relay 209 is then deenergized bythe lifting of the weld electrode assembly and concurrent opening ofswitch 173; the contact thereof again interconnects capacitor 211 withpoint 212; and the cycle is completed.

The system of FIGURE `l0 is shown schematically in FIGURE 11 with likereference numerals representing corresponding parts. In addition to thesli-unt 213 which is shown as comprising a transistor 227 having aresistance 229 connected in its emitter circuit, the primary winding 205has connected across it a transient-suppressing diode 231 and a Zenerdiode 233. Pulse weld amplifier 219 is illustrated as comprising a pairof transistors 235 and 237 interconnected to the welding tips 77 =bymeans of a coupling capacitor 239 and al resistance biasing networkcomprising resistors 241 and 243. The output of this amplifier is takenacross an output resistance 244 interconnected between the collector oftransistor 237 and the negative terminal of a 12 v. D.C. power supplyindicated at 246. It will be appreciated that the second stage of thisD.C. amplifier functions as an emitter follower to provide ia lowimpedance output to the input of the voltage comparator.

The voltage comparator 221 consists of a pair of interconnectedtransistors 245 and 247 forming a differential amplifier circuit. Thebase of transistor 245 constitutes one input to the voltage Icomparatorinterconnected with the output of the D.C. ampliler, while the base oftransistor 247 is connected to the rotor or tap of a potentiometer 249which constitutes the reference voltage gen'- erator 223. The fixedresistance of potentiometer 249 is interconnected by means of aresistance 251 and a second resistance 253 across the terminals of powersupply 246. The right side of the differential amplifier, i.e.,transistor 247, is driven into conduction by the reference voltage, andthe left side remains nonconducting as Ilong as the output of the weldpulse amplifier does not exceed the preestablished value. When thisoutput exceeds the reference voltage, transistor 245 conducts, causingan output or error signal to appear at the collector of transistor 245.The output is applied to the input of the error amplifier whichcomprises two transistors 255 and 257 with an emitter follower stage 259in the output. The output of this error amplifier, taken at the emitterof transistor 259, is coupled to the base or control electrode of theshunt transistor 227 to control its conductivity and thereby control thelevel of the pulse applied from the capacitor 211 to the primary of thewelding transformer 201.

It should be noted that while shunt.213 is illustrated as comprising asingle transistor 227, a plurality of transistors having commonlyconnected bases could be employed to reduce individual collectorcurrents.

In addition to the dynamic control of the weld pulse which takes placeduring a Welding cycle, it is desirable in many instances to provide fora pre-resistance test, i.e., for Ia resist-ance measurement of theworkpiece prior to the initiation of the welding cycle -to determinewhether the resistance of the workpiece is within a certain tolerancerange preselected by the operator. It will be understood that if theresistance is outside of this range, no matter how accurate the dynamiccontrol is, a satisfactory weld joint cannot be assured. It is alsodesirable to conduct a post-resistance testl subsequent to the weldcycle to determine whether the resistance characteristics of theworkpiece indicate a satisfactory weld joint.

The system of FIGURE 12 includes components for performing both apre-resistance test to determine whether the weld should be carried outupon impression of the proper pressure between the weld tips and theworkpiece, and a post-resistance test to determine whether theresistance of the weld point after a welding cycle is within apreselected range of values.

The system of FIGURE l2 includes all of the components illustrated inFIGURE l1, which components provide identical functions in this FIGURE12 system. 4In addition to these, the FIGURE 12 system includes a testcurrent ygenerator 261 which operates at a relatively high frequency,for example, 25 kc. The output o'f this generator is connected acrossthe weld tips 77, for example by connection Ito the bus bars 177 (seeFIGURES 1 and 2). The leads 101, in addition to being interconnected tothe input of the weld pulse amplifier 219, are connected with the inputof Ian amplifier 263. This is a loosely tuncd amplifier operative in the25 kc. operating region. The output of amplifier 263 is applied to adetector 265, for example, a conventional full-wave bridge-typedetector, which supplies a D.C. output voltage proportional to theamplitude level of the 25 kc. signal applied from amplifier 263. Theoutput of detector 265 is in turn coupled to the respective Iinputs oftwo Schmitt trigger circuits 267 and 268. The former forms a part of`the pre-resistance control; while the Ilatter fuuctions to carry out apost-resistance comparison. A D.C. reference voltage from a source 269is applied to the second input of trigger circuit 267. Similarly, thesecond `input of circuit 26S receives a D.C. reference Voltage from agenerator 270. The output of circuit 267 is applied to the coil of arelay 271. This relay has a first set of contacts 271a and a second setof contacts 2'71b, the latter being interconnected with a relay powersupply 273 and the coil `of relay 209. The weld firing switch 173 isshown as having its movable contact interconnected 'with power :supply273, one pole being connected to the coil of relay 209 and the otherpole connected Ito a valveholdintg relay 275. The movable contact ofrelay 275 is connected to a post-Weld test relay 277, the coil :of whichis selectively energized by the loutput of trigger circuit 268. Themovable arm of relay 277 and the arm of contacts 271a are -connected toa 25 v. D.C. source connected 'at point 279.

When employed in the FIGURE 12 system, pulse weld amplifier 219 includesa low-pass filter in its input stage to block the 25 kc. signal fromgenerator 261. It will be appreciated that the primary winding 203presents a high impedance, for all practical purposes, to lthis 25 kc.signal.

In the FIGURE 12 system, a solenoid actuated valve 281 is connected inhydraulic pressure line 145 between the hydraulic cylinder 125 andcylinder 11. This valve is controlled by solenoid 283 wherebywhen thissolenoid is energized the valve closes thereby blocking all motion ofthe yfoot pedal 113 or the vertically movable bracket 21 which carriesthe weld electrodes 77.

Operation of the FIGURE 12 system is Vas follows: Initially solenoid 233is deenergized and fa welding cycle is initiated by the depression offoot pedal 109 which brings weld electrodes intoV contact with theworkpiece. Upon a predetermined pressure between the electrodes and theworkpiece, the Weld firing switch 173 is actuated as explained above. AWeld cycle is thereby initiated, and as explained above in connectionwith FIGURE 10, the feedback circuit consisting of pulse weld amplifier219, the voltage comparator 221 and error amplifier 225 function toflatten out or clip the pulse applied to the workpiece t-o maintain thevoltage across the workpiece 'at a preestablished level. Before a weldis initiated, however, the pre-resistance welding test serves to preventa weld cycle altogether if proper res-istance characteristics are notexhibited by the workpiece. The test current -generator 261 functions asa constant current source to supply a constant current at 25 kc. to theVworkpiece. The voltage across this workpiece (which is la function ofthe resistance thereof) is applied to the input of amplifier 263 Whereit is amplified, and the resulting signal detected by detector 265. Theoutput of the detector is coupled t-o the input of a Schmitt triggercircuit 267. This cir-cuit functions to compare this input with .thethreshold level from generator 269, and if the input is too high(indicating an excessive resistance between the weld tips) the triggercircuit provides an output which energizes the coil of relay 271. Thisin turn actuates contacts 271b to open the weld fire circui't, therebypreventing energization of the coil of relay 209. As a result actuationof the contacts of relay 209 is prevented and a weld cycle cannot 'beinitiated. This signals `the operator that the resistancecharacteristics of the workpiece are such that a satisfactory weld jointcannot be assured. The operator can then take whatever corrective actionmay be necessary.

Actuation of weld firing switch 173, in addition to initiating a weldingcycle, lalso causes deenergization of relay 275, thereby permitting thecontact thereof t-o return to the position illustrated in FIGURE 12.This completes a circuit from point 279 through Ithe contacts of relay277 (an-d falso through contacts 271a of relay 271), through thenow-closed contact of relay 275 to coil 283. This energizes the latter,closes valve 281 and thereby locks the Weld electrodes 77 in their lowerposition.

The gap resistance, i.e., the resistance across the workpiece, increasesappreciably upon welding, for example to a value which is double thepre-weld gap resistance. The post-weld resistance test provides adetermination (1) whether this increase in resistance has occurred, and(2) whether the post-weld resistance is still below a preselected value,indicating a satisfactory weld. Unless both conditions (1) and (2) aresatised, solenoid 283 remains energized, indicating an unsatisfactoryweld.

To determine whether condition (1) is satisfied, trigger circuit -267 isagain brought into play. If the resistance after the weld exceeds thelevel set in by referencevoltage generator 269, trigger circuit 267will, as before, energize relay 271 and actuate contacts 271a to openone of the conductive paths between point 279 and the movable contact ofIrelay 275. If the post-weld resistance is above a level established bythe setting of generator 270, trigger circuit 268 will apply an outputsignal to relay 277. This indicates that condition (2) is not satisfied.Energization of relay 277 maintains coil 283 energized through thecontacts of relay 275. However, if the post-weld resistance is less thanthe value established by generator 270 (i.e., if condition (2) issatisfied), relay 277 will not be energized and the second or remainingconductive path between point 279 and relay contacts 27-5 will beopened. This in turn deenergizes coil 283, opening valve 281 and therebypermitting bracket 21 to move upward, positioning electrodes 77 in theirupper position. Concurrently, switch 173 is actuated to the positionillustrated in FIGURE 12. This reenergizes relay 275 drawing the movablearm out of circuitclosing relation with coil 283.

Summarizing, after initiation of a weld cycle, valve 281 is closed andis automatically reopened only if the post-Weld resistance is (1)greater than the preresistance maximum level as established by thesetting of generator 269, and (2) less than a level established by theadjustment of generator 270. Valve 281 is provided with a manualoverride to permit manual opening of the valve in the event thepost-weldresistance does not satisfy conditions (1) and (2) above.

In the system of FIGURE 12, a first voltmeter V1 is shown interconnectedat the output of the reference voltage generator 223. This meterprovides a means of *selecting a proper clipping level or top to thewelding pulse, i.e., a preselected maximum voltage level for the weldingpulse. A second meter V2 is selectively interconnected by means of athree-position switch either to the output of detector 265, or theoutput of the threshold level generators 269 or 270. In addition toproviding a means of accurately selecting the respect-ive thresholdlevels set in by generators 269 and 270, this meter provides a readingof the resistance exhibited by the workpiece between the weld tips 77,both during the preresistance and the post-resistance test.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

Asvarious changes could :be made in the above constructions and circuitswithout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. A step-welding mechanism comprising a first guide, a carrier slidableon the first guide, a pair of guide means on the carrier parallel to thefirst guide, a pair of electrodes, arms oppositely slidable on saidguide means and rotatable thereon, each arm carrying an electrode, acompensating lever pivoted to said carrier between said arms, the endsof said lever being pivoted to the arms respectively to effect oppositerelative movements therebetween on the carrier, an adjusting screwconnected transversely to the carrier for rotary but nonaxial movementstherewith, said screw having threads of opposite pitches, nuts on saidthreads, said nuts having connections with said arms adapted to preventrotation of the nuts relative to the arms but permitting oppositesliding motions of the armswith respect to the screw, and means forturning the screw to move the arms through the nuts.

2. -A step Welder comprising a first guide, a carrier movable on saidguide, a pair of guide means on the carrier, a pair -of electrodes, armsoppositelymovable on said guide means, each of said arms carrying one ofsaid electrodes to and vfrom work to be welded, a cornpensating deviceon said carrier having means articulated with said arms respectively toeffect opposite relative movements thereof when the carrier is advancedon said first guide and one of the electrodes contacts a portion of workin one plane, whereby the other electrode advances to contact a portionof work in another plane, said arms ibeing articulated with said guidemeans to provide, in addition to said opposite movements, recessive andapproach movements between the a-rms, and adjustable means controllingthe last-named movements.

3. A step welder comprising a rst guide, a carrier movable on saidguide, a pair of .guide means on the carrier, a pair of electrodes, armsoppositely movable on said guide means, each of said arms carrying oneof said electrodes, a compensating lever pivoted to said carrier betweensaid arms, the ends of said lever being articulated with the armsrespectively to eiect opposite relative movements thereof when thecarrier is advanced on said rst guide and one of the electrodes contactsa portion of work in one plane, whereby the other electrode advances tocontact a portion of work in another plane, said arms being articulatedwith said guide means to provide, in addition to said oppositemovements, recessive and approach movements between the arms, androtatable means on the carrier having driving connections with the armsrespectively for etecting said recessive and approach movements.

4. A step welder according to claim 3, wherein said rotatable means is ascrew having oppositely threaded ends and each of said drivingconnections is a nut on one of the `screw threads, each nut having anonrotary and lost-motion connection with one arm to provide for saidopposite movements of the arms on the carrier.

5. A step Welder according to claim 4, wherein said electrodes haveparallel faces between them.

r6. A step Welder comprising a first guide, a carrier movable on saidguide, a pair of guide means on the carrier, a pair of electrodes, armsoppositely movable on said guide means, each of said arms carrying oneof said electrodes, insulating means preventing flow of current betweensaid electrodes through said carrier, a compensating lever pivoted tosaid carrier between said arms, the ends of said lever being articulatedwith the arms respectively to effect opposite relative movements thereofwhen the carrier is advanced on said rst guide and one of the electrodescontacts a portion of work in one plane, whereby the other electrodeadvances to Contact a portion of work in another plane, insulating meanspreventing flow of current between said electrodes through saidcompensating lever, said arms being articulated with said guide means toprovide, in addition to said opposite movements, recessive and approachmovements between the arms, rotatable means on the carrier havingdriving connections with the arms respectively for effecting saidrecessive and approach movements, and insulating means preventing ow ofcurrent between said electrodes through said rotatable means.

References Cited by the Examiner UNITED STATES PATENTS 2,394,822 2/1946Teplitz 219-86 2,588,062 3/ 1952 Vorderstrasse 219-56 2,951,932 9/1960Heckman et al. Q 219-78 2,979,599 4/ 1961 Width 219-87 FOREIGN PATENTS462,729 3/ 1937 Great Britain.

ANTHONY BARTIS, Acting Primary Examiner. JOSEPH V. TRUI-IE, Examiner.

1. A STEP-WELDING MECHANISM COMPRISING A FIRST GUIDE, A CARRIER SLIDABLEON THE FIRST GUIDE, A PAIR OF GUIDE MEANS ON THE CARRIER PARALLEL TO THEFIRST GUIDE, A PAIR OF ELECTRODES, ARMS OPPOSITELY SLIDABLE ON SAIDGUIDE MEANS AND ROTATABLE THEREON, EACH ARM CARRYING AN ELECTRODE, ACOMPENSATING LEVER PIVOTED AT SAID CARRIER BETWEEN SAID ARMS, THE ENDSOF SAID LEVER BEING PIVOTED TO THE ARMS RESPECTIVELY TO EFFECT OPPOSITERELATIVE MOVEMENTS THEREBETWEEN ON THE CARRIER, AN ADJUSTING SCREWCONNECTED TRANSVERSELY TO THE CARRIER FOR ROTARY BUT NONAXIAL MOVEMENTSTHEREWITH, SAID SCREW HAVING THREADS OF OPPOSITE PITCHES, NUTS ON SAIDTHREADS, SAID NUTS HAVING CONNECTIONS WITH SAID ARMS ADAPTED TO PREVENTROTATION OF THE NUTS RELATIVE TO THE ARMS BUT PERMITTING OPPOSITESLIDING MOTIONS OF THE ARMS WITH RESPECT TO THE SCREW, AND MEANS FORTURNING THE SCREW TO MOVE THE ARMS THROUGH THE NUTS.